Therefore the grand, the vital, the most urgent question for his times, as indeed for all times, was, _What is Truth? What is Right_? In the midst of all this variableness and uncertainty of human opinion, is there no ground of certainty? Amid all the fluctuations and changes around us and within us, is there nothing that is immutable and permanent? Have we no ultimate standard of Right? Is there no criterion of Truth? Plato believed most confidently there was such a criterion and standard. He had learned from Socrates, his master, to cherish an unwavering faith in the existence of an Eternal Truth, an Eternal Order, an Eternal Good, the knowledge of which is essential to the perfection and happiness of man, and which knowledge must therefore be presumed to be attainable by man. Henceforth, therefore, the ceaseless effort of Plato's life is to attain a standard (κριτήριον)[506]--a CRITERION OF TRUTH. [Footnote 506: "Theætetus," § 89.] At the outset of his philosophic studies, Plato had derived from Socrates an important principle, which became the guide of all his subsequent inquiries. He had learned from him that the criterion of truth must be no longer sought amid the ever-changing phenomena of the "sensible world." This had been attempted by the philosophers of the Ionian school, and ended in failure and defeat. It must therefore be sought in the metaphenomenal--the "intelligible world;" that is, it must be sought in the apperceptions of the reason, and not in opinions founded on sensation. In other words, he must look _within_. Here, by reflection, he could recognize, dimly and imperfectly at first, but increasing gradually in clearness and distinctness, two classes of cognitions, having essentially distinct and opposite characteristics. He found one class that was complex (σνγκεγυμένον), changeable (θάτερον), contingent and relative (τὰ προς τι σχέσιν ἔχοντα); the other, simple (κεχωρισμένον), unchangeable (ἀκίνητον), constant (ταὐτόν), permanent (τὸ ὂν ἀει), and absolute (ἀνυπόθετον = ἁπλοῦν). One class that may be questioned, the other admitting of no question, because self-evident and necessary, and therefore compelling belief. One class grounded on sense-perception, the other conceived by reason alone. But whilst the reason recognizes, it does not create them. They are not particular and individual, but universal. They belong not to the man, but to the race.